Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about 500 to about 1,000 times its own weight, and each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of various products, for example, hygiene products such as paper diapers for children, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.
In most cases, these super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. For these applications, the super absorbent polymer should exhibit a high moisture absorbency, it should not release the absorbed water even in the external pressure, and additionally it should well retain the shape even in a state where the volume is expanded (swelled) by absorbing water, and thereby exhibit excellent liquid permeability.
In recent years, as the demand for a thin diaper increases, the proportion of the absorbent polymer in the diaper tends to increase. Therefore, the water absorbent polymer needs to have the performance of the fiber material of the diaper. For this, the water absorbent polymer should have a high water absorption capacity as well as a high absorption rate and a liquid permeability.
In the process of producing the water absorbent polymer, it is generally necessary to pulverize the hydrogel prepared by polymerizing the monomer of the water absorbent polymer. The pulverization of the hydrogel is a process required for producing a super absorbent polymer in the form of a powder or a particle, and this process greatly affects the physical properties of the super absorbent polymer.
In this regard, various studies have been undertaken. As an example, Japanese Patent No. 3415036 discloses a preparation method that minimizes damage to hydrogel during pulverization of the hydrogel, in order to reduce a water-soluble component that can induce a reduction in water absorption capacity. However, the above method can achieve a high water absorption capacity, but it is insufficient to obtain the absorption rate at the level recently required for diapers.
As another example, Japanese Patent No. 4866733 discloses a method of using an internal-crosslinking agent in an amount of 0.2 mol % or more and adjusting the perforation diameter of the perforated plate of the screw type extruder during pulverization of the hydrogel, in order to improve the liquid permeability and absorption rate of the super absorbent polymer. However, the method disclosed herein can achieve a high absorption rate and a liquid permeability, but there is a problem that the water absorption capacity is lowered.
For these reasons, there is a continuing need to develop a technique capable of providing a super absorbent polymer having improved water absorption capacity, absorption rate and liquid permeability while maintaining excellent basic absorption performance.